gdbsupport/event-loop.cc: simplify !HAVE_POLL paths

gdbsupport/event-loop.cc throughout handles the case of use_poll being
true on a system where HAVE_POLL is not defined, by calling
internal_error if that situation ever happens.

Simplify this by moving the "use_poll" global itself under HAVE_POLL,
so that it's way more unlikely to ever end up in such a situation.
Then, move the code that checks the value of use_poll under HAVE_POLL
too, and remove the internal_error calls.  Like, from:

    if (use_poll)
      {
  #ifdef HAVE_POLL
        // poll code
  #else
      internal_error (....);
  #endif /* HAVE_POLL */
      }
    else
      {
	// select code
      }

to

  #ifdef HAVE_POLL
    if (use_poll)
      {
        // poll code
      }
    else
  #endif /* HAVE_POLL */
      {
	// select code
      }

While at it, make use_poll be a bool.  The current code is using
unsigned char most probably to save space, but I don't think it really
matters here.

Co-Authored-By: Youling Tang <tangyouling@loongson.cn>
Change-Id: I0dd74fdd4d393ccd057906df4cd75e8e83c1cdb4